There are at least three different opioid receptors (μ, δ and κ) that are present in both central and peripheral nervous systems of many species, including humans. Lord, J. A. H., et al., Nature, 1977, 267, 495. Activation of the δ opioid receptors induces analgesia in various animal models. Moulin, et al., Pain, 1985, 23, 213. Some work suggests that the analgesics working at δ opioid receptors do not have the attendant side effects associated with μ and κ opioid receptor activation. Galligan, et al., J. Pharm. Exp. Ther., 1985, 229, 641. The δ opioid receptor has also been identified as having a role in circulatory systems. Ligands for the δ receptor have also been shown to possess immunomodulatory activities. Dondio, et al., Exp. Opin. Ther. Patents, 1997, 10, 1075. Further, selective δ opioid receptor agonists have been shown to promote organ and cell survival. Su, T-P, Journal of Biomedical Science, 2000, 9(3), 195-199. Ligands for the δ opioid receptor may therefore find potential use as analgesics, as antihypertensive agents, as immunomodulatory agents, and/or agents for the treatment of cardiac disorders.
Numerous selective δ opioid ligands are peptidic in nature and thus are unsuitable for administration by systemic routes. Several non-peptidic δ opioid receptor ligands have been developed. See, for example, E. J. Bilsky, et al., Journal of Pharmacology and Experimental Therapeutics, 1995, 273(1), 359-366; WO 93/15062, WO 95/04734, WO 95/31464, WO 96/22276, WO 97/10216, WO 01/46192, WO 02/094794, WO 02/094810, WO 02/094811, WO 02/094812, WO 02/48122, WO 03/029215, WO 03/033486, JP-4275288, EP-A-0,864,559, U.S. Pat. Nos. 5,354,863, 6,200,978, 6,436,959 and US 2003/0069241.
While there are a large number of non-peptidic δ opioid receptor modulators, there is still an unfulfilled need for compounds with selective δ opioid receptor activity that may be used in methods to provide beneficial pharmaceutical characteristics while minimizing undesirable side effects. The present invention is directed to these, as well as other important ends.